1. Field of the Invention
The invention relates to a compact controlled twin-flow or dual-flow turbocharger, preferably twin-flow, having an integrated bypass. The invention more particularly concerns a turbocharger with an improved rotary valve design for regulating gas flow, including bypass.
2. Description of the Related Art
Reciprocating internal combustion engines have long been equipped with turbochargers. In a simple uncontrolled fixed-nozzle turbocharger system, the maximum charging pressure is a function of the strength of the engine. The uncontrolled turbocharger must thus be so designed that the optimal performance is reached only at high engine speeds. Unfortunately, at all other speed regions the turbocharger provides suboptimal boost or air volume.
Controlled turbochargers provide improved performance, in that the turbine optimal operating point is already reached at low or medium engine speeds. In a simple controlled system, when the flow rate of exhaust gases increases and the turbocharging pressure becomes too high, part of the exhaust gases are simply discharged into the surrounding atmosphere through a wastegate so as to bypass the turbine, whereby damage to the engine due to the excessive boost at high speeds can be avoided. However, since exhaust gases bypass the turbine, energy losses are high and the engine performance drops at high speeds.
Where there is sufficient engine compartment space it is known to install two turbochargers—a LP and an HP turbocharger—in series. At the lower speed range of the internal combustion engine the HP stage and LP stage are operated in series, and as the rotational speed of the internal combustion engine increases, a changeover can be made to single-stage compression exclusively in the low-pressure (large volume) compressor, partially or completely bypassing the high-pressure turbine. While such a system provides greatly improved efficiency, it lacks compactness.
It is also know to provide multiple flow conduits within a single turbocharger casing, such that the single turbocharger casing can be controlled to perform alternately as LP and as HP turbine. These casings can be classified into twin-flow casing (zwillingsstrom-gehaeuse) and dual-flow casing (doppelstromgehaeuse) type.
In a twin-flow casing the spiral turbine casing is divided by at least one radial partition into two axially adjacent spirals. The exhaust gas of each spiral enters the turbine wheel inlet to impact the entire periphery (360°) of the turbine wheel, with axially adjacent spiral conduits impacting axially adjacent segments of the turbine wheel.
In a dual-flow casing spirals or half spirals are so arranged that exhaust gas from each spiral acts upon the entire width of the turbine wheel inlet, but only over half the periphery (180°) of the turbine wheel.
The spiral selection or operation is controlled via a gate valve (throttle valve, flap, slide valve) which enlarges flow cross section with increasing supercharger speed. A control device is generally provided with sensing means for sensing boost pressure or speed, and an adjustment member for actuating the gate valve.
In addition to the above design considerations, turbochargers for multi-cylinder reciprocating internal combustion engines have to be designed to avoid exhaust interference. In order to overcome this problem, which occurs especially when the boost is lower than the exhaust gas pressure or the engine is partially loaded, supercharging systems are known in which the exhaust manifold to the turbine wheel is divided into two or more branches. The two exhaust gas lines are often brought from the cylinders of the engine to the spiral casing of the turbine, one group of cylinders, respectively, always being connected to an exhaust gas line in accordance with the firing sequence of the engine, so that cylinders in adjacent explosion order will not exhaust into the same branch and consequently exhaust interference can be avoided.
To utilize the energy contained in the exhaust gas optimally, each of the two exhaust gas flows should be conducted to the inlet of the turbine wheel with as little influence as possible being exerted thereon by the other. This should primarily avoid backfiring of the exhaust gas flow of either exhaust gas line, which arrives at the turbine and pulsates in dependence upon the opening times of the exhaust valves, into the other of the exhaust gas lines when no appreciable gas flow is flowing in the other exhaust gas line at the same time. The exhaust gas pulses should strike the turbine blades with full kinetic energy and, therefore, they must not encounter a large connected-line volume.
The same twin- or dual-flow casing designs discussed above can also be used specifically to avoid exhaust interference and take advantage of exhaust pulses. See U.S. Pat. No. 3,614,259 (Neff) teaching a divided turbine casing which may be used to provide either a pulse turbine or a variable speed turbine, with gas flow controlled via a flapper valve. In the case of an impulse turbine multiple exhaust gas lines are coupled to the turbine casing, thus the gate valve or flow control means must be of commensurate complexity, controlling flow through two or four or more flow paths.
See also U.S. Pat. No. 4,389,845 (Koike) teaching a triple flow turbine casing, wherein a control valve means is provided adjacent to first and second gas inlets (separated by a partition wall) so that the exhaust gases can flow only through first (double) scrolls or both through the first (double) scrolls and a second (single) scroll. The control valve means is operatively connected through a linkage to a control unit which consists of a diaphragm, which is deflected in response to the boost pressure, and a bias spring. In response to the boost pressure, the control valve means is so operated as to close or open the second scroll. More specifically, when the engine is running at low or medium speeds or is partially loaded, the flow rate of the exhaust gases is low so that the boost pressure is low. Therefore the control unit so operates as to close the second scroll. As a result, the exhaust gases flowd into the first scrolls only. Since the first scrolls are substantially separated from each other, no exhaust interference results. In addition, the pulsations of the exhaust gases can be utilized very effectively. When the second scroll is closed by the control valve means, the cross sectional area of the scroll structure is less than that when the second scroll is opened, so that the velocity of the exhaust gases flowing through the first scrolls is increased, whereby the sufficiently high boost pressure is attained. When the engine is running at high speeds, the flow rate of the exhaust gases increases, so that the control unit causes the control valve means to open the second scroll and, consequently, the overall cross sectional area of the scroll structure increases. The exhaust gases flowing from the separate gas inlets are mixed or diffused in the second scroll downstream of the control valve means. Since the second scroll exhibits less resistance to the flow of the exhaust gases, the boost pressure can be maintained at a predetermined level or magnitude.
As will be readily apparent from the above, turbine casings are relatively advanced with respect to the design of the channels for feed of exhaust gases from the internal combustion engine to the turbine. The present invention is not concerned with the turbine casing design. The above references teach the environment in which the present invention can be used, and this teaching is incorporated herein by reference.
The present invention is more specifically concerned with the control valve used within the casing for selection or operation of the flow spirals and/or enlargement or constriction of flow cross-section.
As illustrated in U.S. Pat. No. 4,389,845, the control valve may be a flap type valve (FIG. 6) or a slide type gate valve (FIG. 11). See U.S. Pat. No. 4,443,153 (Dibelius) and U.S. Pat. No. 4,351,154 (Richter) teaching dual exhaust manifolds, with each exhaust line subsequently divided into two or more conduits leading to the turbocharger. Flow is controlled via gate or flap valves.
An improvement in valve design can be seen in U.S. Pat. No. 4,544,326 (Nishiguchi et al.) which teaches, in addition to flapper type valves (FIG. 6A), a rotary valve (FIG. 8A, 9A, 10A, 11A, 13, 16). The rotary valve has a L-shaped cross section, as shown in FIG. 10B. The rotary valve has a solid segment portion whose cross sectional shape is a segment of a circle, and a circular end which has a circular disc shape and to which the shaft of the rotary valve is fixed. The rotary valve has one circular end only. The other side of the rotary valve is open. A cavity of the rotary valve is bounded by the circular end on one side, but the other side of the cavity is open. The open side of the cavity faces the partition wall of the turbine casing. The partition wall is formed with a hole. The hole opens into the cavity. In the closed position shown in FIG. 10A, the hole makes a fluid communication between a main scroll passage and a secondary scroll passage at a position upstream of the solid segment portion of the rotary valve. This fluid communication through the hole is maintained even in the valve open position. The hole can relieve dynamic pressure acting upon the cavity bottom when the valve is in the closed position.
This solid segment, whose cross sectional shape is a segment of a circle, is offset from the rotary valve cylinder center axis of rotation. Thus, any pressure applied to the solid segment causes the valve to be unbalanced, increasing the force required to rotate the valve between positions, reducing valve speed, and increasing the likelihood of valve sticking.
Further, in the case that there is a requirement to include a bypass function in the turbocharger, this is accomplished using additional control means, and adds to the space requirement and complexity. There is a need for a simpler and more efficient design for the bypass function.